1. Field of the Invention
The present invention relates to storage and retrieval of high density information and more particularly to optical disk methods, media and systems characterized by predetermined record structure (comprising high-fractional-bandwidth, varying-length pit regions) and predetermined detection techniques.
2. Description of the Prior Art
Optical disk technology for writing and reading with high information densities has been in active development in recent years. One general application of this technology is in recording and playing back FM encoded video (e.g. television) signals. Most optical disk approaches involve minute marks and spaces formed along spiral or concentric information tracks. The tracks are scanned by a reading light beam and the read light, as modulated by the marks and spaces, passes to a photodetector which outputs a generally-corresponding, modulated electrical signal.
The range of frequencies used to FM encode a video signal usually does not result in optical disks having "fractional bandwidths" greater than about 4.0 (The term "fractional bandwidth" of an optical disk is defined herein to be the ratio of the maximum mark length on the disk, in-track, to the minimum mark length on the disk.) Readout techniques for such FM encoded information usually detect the in-track mark-to-mark spacings to derive a sinusoidal FM electrical signal (in contrast to detecting the precise location of the leading and trailing edge of each successive mark). The detected FM video signal is then demodulated (essentially by averaging the spacings between two or more contiguous pairs of FM signal crossings) to produce, e.g., an analog basehand television signal.
The optical disk technology also holds strong potential for massive digital data storage, e.g. as a memory media for digital computers. However, the prior art optical disk systems and methods for storing and retrieving television type video information are not optimal for the mass digital data storage applications. In the latter applications it is important to maximize the information bit densities of the disks and to minimize the instances of reading a wrong digital bit of information from a recorded disk (i.e., minimize the bit error rate). It has been found that the more efficient techniques for encoding digital data (e.g. long run length transition modulation codes) presuppose optical disk fractional bandwidths in excess of 4.0 (for constant angular velocity recording). Also in such applications it is advantageous, from the viewpoints of maximizing packing density and minimizing bit error rate, to detect the precise location of leading and trailing pit edges.